Ok here is the situation:
I was messing around and turn on all the function in the Authentication Configuration but without config in detail. Then when I restart the computer, I can't log in anymore using the same user name and password.
I tried the root username and the other username I have, both doesn't work. However, I know that it has nothing to do with entering an incorrect password because it says Authentication Fail but not incorrect username and password combination in black letter.
By the way, I am using the RedHat Linux 9.1 .So anyway to fix this without reinstalling a new Linux again? Any advise will be helpful to me. Thanks in advanced.

well for security reasons linux doesn't keep a password in a file to compare it to the string you type in. Your password is encyrted in a file i think called shadow or something like that. So when you try to login it compares the encrypted value of what you entered with the encrypted password in shadow to aunthenticate it. So maybe it does have to do with wrong password, i don't know for sure. Maybe the shadow file got messed up some how. I know this isn't really helpful but it is all i could think of. And at the very least it got your post bumped.

quote: " I was messing around and turn on all the function in the Authentication Configuration but without config in detail."

also this seems to be where things went wrong but to me seems a little vague. Exactly how and what were you messing around with? Maybe if you can be more specific with what you were doing then someone might be able to help you better.

In the Authentication Configuration Screen, I simply check(turn on) all the functions. However, for each one of the function that has configuration option, I didn't change anything at all. I think it's very important to let you know that the Linux i am using is RedHat 9.0.
Why I think the login failure has nothing to do with entering the incorrect password because I also tried to enter a wrong password intentionly just to see if it has the same result but no, it will warn me that I have an incorrect username/password combination instead of "Authentication Fail" pop up window.
Please help, I need the linux for My project. thank you.

Still stuck, anyone can help me out?

You could boot up with cd#1 in rescue mode or the Fedora rescue cd or any linux live cd. Mount the root partition and then chroot into it. Then run the comman: sys-unconfig . That will quickly reboot the system whereby you will need to re-enter the root password and some other settings.
man sys-unconfig

Thanks for you help. But when I tried to follow the procedure you say and typed in man sys-unconfig it says "No manual entry for sys-unconfig" , and when I typed in sys-unconfig it says "System halted". By the way, I already use chroot to mount my root system back.

Any solution to that?

When I said man sys-unconfig , I meant that you could read up on it once you have a working Redhat / Fedora system.
When you run that command ( sys-unconfig ) from a live cd in a chroot environment, the message " system halted" maybe all you get. Did you try to reboot into the system after that?

Wow! It works! You are good man! Now I can have my system back without erasing everything. Thanks!

You'r welcome!
Don't forget your date/time will be off.